TEHRAN (Reuters) - The head of Iran's Revolutionary Guards on Monday vowed to "retaliate" against the United States and Britain after accusing them of backing the perpetrators of a suicide bombing that killed six Guards commanders.

Iranian media say the Sunni Muslim insurgent group Jundollah (God's soldiers) has claimed responsibility for Sunday's bombing in Sistan-Baluchestan province, which killed 42 people in all.

The incident threatened to overshadow talks between Iran and global powers in Vienna on Monday intended to tackle a standoff about Iran's nuclear ambitions.

"Behind this scene are the American and British intelligence apparatus, and there will have to be retaliatory measures to punish them," Jafari was quoted as saying.

Jundollah, which has been blamed for many attacks since 2005 in the desert province bordering Pakistan, says it is fighting to end discrimination against Sunni Muslims by Iran's dominant Shi'ites. Its leader is Abdolmalek Rigi.

"This person himself and his plans are undoubtedly under the umbrella and the protection of these (U.S., British and Pakistani) organizations," Jafari said.

"TRAINED BY U.S. AND BRITAIN"

Iranian television quoted General Mohammad Pakpour, commander of the Guards' ground forces, whose deputy was killed in the bombing, as saying:

"The base of the terrorists and rebels has not been in Iran. They are trained by America and Britain in some of the neighboring countries."

The United States, Pakistan and Britain have all condemned the bombing, the bloodiest attack in Iran since the 1980-88 war with Iraq, and denied involvement.

"We reject in the strongest terms any assertion that this attack has anything to do with Britain," said a spokeswoman at Britain's Foreign Office. "Terrorism is abhorrent wherever it occurs."

The bombing of a mosque in Zahedan, capital of Sistan-Baluchestan, reportedly also claimed by Jundollah, killed 25 people in May.

The underdeveloped desert province, mostly populated by Sunni Muslims, borders both Pakistan and Afghanistan and has frequently been the scene of clashes between security forces, ethnic Baluch Sunni insurgents and heavily-armed drug smugglers.

The victims of the bombing in the city of Sarbaz included two employees of the state broadcaster IRIB, the company said, and number of tribal chiefs who were due to hold a meeting with the Guards to promote Shi'ite-Sunni unity. The Guards said the attack was aimed at fomenting sectarian strife.